This might be from an old version of ruby, but:

irb(main):001:0> a
NameError: undefined local variable or method `a' for main:Object
        from (irb):1
        from C:/Ruby23-x64/bin/irb.cmd:19:in `<main>'
irb(main):002:0> b
NameError: undefined local variable or method `b' for main:Object
        from (irb):2
        from C:/Ruby23-x64/bin/irb.cmd:19:in `<main>'
irb(main):003:0> a=b
NameError: undefined local variable or method `b' for main:Object
        from (irb):3
        from C:/Ruby23-x64/bin/irb.cmd:19:in `<main>'
irb(main):004:0> a
=> nil

This NameError doesn’t stop the assignment, going through despite the error.